Historically, displays were manufactured to meet the requirements of a video color standard and the video color standard was developed based on the feasibility of manufacturing such a display. A display might be required to display two different video types, for example, High Definition (HD) defined by the International Telecommunications Union-Radio Communications (ITU-R) Recommendation BT.709, or standard definition as defined by ITU-R Recommendation BT.601. The differences between the two standards were minor and typically handled by a simple 3×3 matrix conversion.
However, with advent of digital photography, additional color standards arose that allowed the display of a wider range of colors (wide gamut), and manufacturers created displays that also displayed a wide gamut. One method of displaying a wide gamut employs Organic Light Emitting Diode (OLED) displays.
Both OLED and LCD displays share a similar active matrix architecture. The display will have an integrated circuit (display driver IC or DDIC) that converts the Red-Green-Blue (RGB) digital input and drives a voltage or current to an active circuit element at each pixel. The active circuit element will keep that voltage or current active for the pixel until the next time it updates. Crosstalk can result because of coupling between the traces connecting the display driver to the pixels or because of interactions between the color subpixels. Crosstalk decreases the color accuracy making it an unwanted attribute of a display.
Unfortunately, OLED displays require more complex circuitry than an LCD display to produce an image, and sampling and keeping an accurate current value is more difficult than a voltage. Therefore, non-linear cross talk can exist between the color channels. Because of the non-linearities in the cross talk, calibration of OLED displays requires a larger number of color measurements than an LCD display for accurate results in calibration.
Current methods for calibrating a display for different color standards involve measuring the white and pure RGB colors in terms of the XYZ values. The methods use the XYZ values because they are device invariant and provide a quantitative link between the spectral distribution of the color the physiological perception of the color. The methods normalize the measurements to create xy.x=X/(X+Y+Z)y=Y/(X+Y+Z).
The xy coordinates allow calculation of the correct color space conversion matrix from a device independent color in this case XYZ, to a device dependent color, such as RGB. All the color standards are described using a set of xy coordinates for RBG and W that allows the calculation of the color space conversion matrix to go from the color standard RGB values to the device independent XYZ colors. The combination allows one to convert from the input color standard RGB value to the display RGB value so that regardless of the display gamut the correct colors are viewed.
Another aspect of the color standard and the displays involves the shape of the non- linear function between the RGB values at the input and the amount of RGB light emitted from the display. The Electrical to Optical Transfer Function (EOTF) also needs to be matched between the display and the standard for the image to look correct. Because of this, the standard will specify an EOTF based on a variety of factors and it is up to the display to make sure what is implemented matches the standard so that the image looks correct
Typical methods for the calibration of panels take hundreds of measurements and then interpolate between those measurements without considering the properties of the panel or the display technology. This takes time and has too high of a cost of implementation on the production line. A need exists to provide accurate calibration of displays with fewer measurements on a production line.